Temperature control system

ABSTRACT

A temperature-sensing thermistor and a heating element are positioned in a region of temperature to be controlled. The thermistor is connected in a bridge circuit, the output of which charges a capacitor. The voltage on the capacitor controls the firing of a thyristor which passes current to the heating element. Means are provided for discharging the capacitor periodically.

United States Patent lnventor Harold M. Neer [fi -Eh Cited A I No gm'gUNITED STATES PATENTS P 3,275,802 9/1966 Vandivere etal 219/499 FiledMay 18, 1970 3,381,226 4/1968 Jones et al. 219/501 Patented Jan. 4, 19721 3 46 8 9 Assignee Phillips Petroleum Company 7, 17 [1969 er 219/501Primary Examiner-Bernard A. Gilheany Assistant ExaminerF. E. BellTEMPERATURE CONTROL SYSTEM mmmey Young and Quigg 5 Claims, 1 DrawingFig.

U.S. Cl. ABSTRACT: A temperature sensing thermistor and a heatingelement are positioned in a region of temperature to be con- Int. Cl1105b 1/02 trolled. The thermistor 18 connected m a bridge circuit. theField of Search 219/501,

499 output of WhlCh charges a capacitor. The oltage on the capacitorcontrols the firing of a thyristor which passes current to the heatingelement. Means are provided for discharging the capacitor periodically,

rllu rllu II b llb L [I2 a 14 4 yv 17 E 52 2| 22 2 2745 1 -48 I I2c 20 1lab I a b E as g 55 69 Ease TEMPERATURE CONTROL SYSTEM It is commonpractice to control various types of processes in response to outputsignals of analytical instruments. Many of these analytical instrumentsmust be maintained at constant temperatures in order to provide reliablemeasurements. This is usually accomplished by positioning the instrumentin an insulated housing and regulating the temperature within thehousing in response to a measurement of the temperature therein. Varioustypes of temperature control systems are well known in the art foraccomplishing this result.

In many industrial operations it is necessary to position the analyticalinstrument at a location which is remote from the control equipmentutilized to regulate the temperature of the instrument housing. This isparticularly true where there is a danger of explosions from electricalsparks and where space is at a premium. In operations of this type it isnecessary to connect the temperature-sensing element to the controlcircuit by electrical leads which may be of substantial length. This canresult in the generation of extraneous signals, particularly when theleads are located in the vicinity of other electrical equipment.

In accordance with this invention, there is provided an improvedtemperature control system which is particularly adapted to be used whenthe sensing element is positioned a substantial distance from theremainder of the circuit. The control system of this invention utilizesa bidirectional triode thyristor (triac) to control the application ofheating current in response to a measured temperature. The passage ofcurrent through the triac is regulated in response to the chargeaccumulated on a capacitor. The temperature-sensing element is connectedin a bridge circuit, the output of which is amplified to control therate at which the capacitor is charged. The capacitor is quicklydischarged during each cycle of voltage applied across the triac so thatthe charging of the capacitor starts from a constant level during eachcycle.

The accompanying drawing is a schematic circuit diagram of apparatusconstructed in accordance with this invention.

Referring now to the drawing in detail, there is shown a housing whichcontains a temperature-sensitive resistance element 11 and an electricalheating element 12. Housing 10 can also contain an analytical instrumentor other equipment, not shown, which is to be maintained at a constanttemperature. Electrical leads 11a and 11b serve to connect resistor 1 1to the control circuit of this invention, and leads 12a and 12b connectheater 12 to the control circuit. Housing 10 can be positioned asubstantial distance from the remainder of the control circuit, althoughthis is not necessary.

The control circuit is energized by a source of alternating current 13which is connected across terminals 14 and 15, the latter being theneutral lead, returned to ground elsewhere, in the power system. Heaterl2 and a triac 54 are connected in series relationship between terminals14 and 15 so that triac 54 controls the current flow through heater 12to regulate the temperature of the interior of housing 10. The circuitemployed to control conduction through triac 54 is in turn controlled bythe resistance of element 11. Lead 11a is connected to one terminal of acapacitor 16, the second terminal of which is connected to ground.Voltage regulating Zener diodes l7 and 18 are connected in seriesrelationship between lead 11a and ground, the junction between thesediodes being designated as terminal 19. A variable resistor is connectedbetween terminal 19 and lead 11b. A diode 50 and a resistor 51 areconnected between lead 11a and power terminal 14. Temperature-sensitiveresistor 11 and variable resistor 20 thus constitute two arms of abridge network.

The junction between resistors 11 and 20 is connected by resistors 21and 22 to the gate terminal of a field effect transistor 23. A firstcapacitor 25 is connected between lead 11a and the junction betweenresistors 11 and 20. A second capacitor 26 is connected between lead 11aand the junction between resistors 21 and 22. The drain terminal oftransistor 23 is connected by resistors 27 and 28 to terminal 19. Acapacitor 29 is connected between the source terminal of transistor 23and lead 1 1a. A resistor 30 is connected between the source terminal oftransistor 23 and the source terminal of a second field effecttransistor 31. A resistor 32 is connected between the source terminal oftransistor 31 and lead 110. Resistors 33 and 34 are connected betweenthe drain terminal of transistor 31 and the tenninal 19. The emitter ofa transistor 35 is connected to terminal 19, and the base of thistransistor is connected to the junction between resistors 27 and 28. Acapacitor 36 and a diode 37 are connected between the collector oftransistor 35 and the source terminal of transistor 23. The collector oftransistor 35 is connected by a resistor 38 to the base of a transistor39. A resistor 40 is connected between the base of transistor 39 and thejunction between capacitor 36 and diode 37. The collector of transistor39 is connected by a resistor 41 to the first terminal of a capacitor55. The second terminal of capacitor 55 is connected to terminal 15. Theemitter of a transistor 42 is connected to terminal 19, and the base ofthis transistor is connected to the junction between resistors 33 and34. A diode 43 is connected between the collector of transistor 42 andthe source of transistor 31. The gate of transistor 31 is connected by aresistor 45 and resistors 46 and 47 to rectifier 50. A resistor 48 isconnected between terminal 19 and the junction between resistors 46 and47. A capacitor 49 is connected between the emitter of transistor 39 andthe junction between resistors 45 and 46.

Resistors 47 and 48 constitute the respective third and fourth arms ofthe bridge network previously mentioned. The transistor circuit thus fardescribed constitutes a differential amplifier which compares thepotential at the junction between resistors 11 and 20 with the potentialat the junction between resistors 47 and 48. Any difference betweenthese two potentials is amplified by the differential amplifier. Theoutput signal of the differential amplifier, which appears at thecollector of transistor 39, serves to charge capacitor 55. As describedin greater detail hereinafter, the potential on capacitor 55 serves tocontrol current flow through triac 54 and thus the amount of heatsupplied by element 12.

Capacitors 25 and 26 and resistor 21, which has a very large value,serve to filter any alternating current signals which may appear acrossleads 1 la and 11b. This is particularly important when housing 10 isremote from the remainder of the circuit and the leads pass in closeproximity to other electrical equipment.

A circuit is provided to discharge capacitor 55 at the end of each halfcycle of applied voltage from source 13. This assures that the chargingof capacitor 55 always starts at the same level. To this end, a resistor52 and a diode 53 are connected in series between terminals 14 and 15.The junction between elements 52 and 53 is connected to the base of atransistor 60. A resistor 61 is connected between the base of thistransistor and terminal 15. A resistor 62 is connected between thecollector of transistor 60 and terminal 19. A diode 63 is connectedbetween resistor 52 and the collector of transistor 60. The collector oftransistor 60 is also connected to the base of a transistor 64 which hasa grounded emitter. The collector of transistor 64 is connected to thebase of a transistor 65 which has a grounded collector. The emitter oftransistor 65 is connected to the junction between a resistor 66 and atrigger diode 68, which can be bilateral trigger diode, type MPT 20. Thesecond tenninal of resistor 66 is connected to capacitor 55. The secondterminal of trigger diode 67 is connected to the control electrode oftriac 54. A resistor 68 and a capacitor 69 are connected in parallelbetween trigger diode 67 and terminal 15.

As previously mentioned, capacitor 55 is charged in response to anunbalance signal of the bridge network which includes resistance element11. When the potential on capacitor 55 equals the breakdown potential oftrigger 67, conduction takes place to fire triac 54. Capacitor 55 isdischarged during each half cycle of applied potential by the circuitwhich includes transistors 65, 64 and 60. When a positive potentialappears at the junction between resistor 52 and diode 53,

diode 63 conducts. This serves to cut off transistor 64 and transistor65. When a negative potential appears at the junction between resistor52 and rectifier 53, transistor 60 conducts. This also serves to cut offtransistor 64 and transistor 65. However, during each half cycle ofapplied potential, when the potential at the junction between resistor52 and rectifier 53 is substantially zero, transistors 64 and 65conduct. This discharges capacitor 55 through resistor 66. Thus, thecharging of capacitor 55 begins from a zero value during each half cycleof applied potential from source 13. The speed at which capacitor 55 ischarged up to the threshold value of trigger 67 determines the durationthat triac 54 is fired during each half cycle of applied potential. Thisin turn regulates the duration of current fiow through heating element12 and thus the temperature of housing 10.

While this invention has been described in conjunction with a presentlypreferred embodiment, it obviously is not limited thereto.

What is claimed is: 1. Temperature control apparatus comprising: anelectrical heating element; a temperature-sensing element adapted to bepositioned in a region of temperature to be controlled; a source ofalternating current; a bidirectional thyristor having a gate electrode;means connecting said thyristor and said heating element in circuit withsaid source of alternating current; a capacitor; circuit means includingsaid temperature sensing element to establish a current, the magnitudeof which is representative of the temperature sensed by said sensingelement; means to apply the current established by said circuit means tosaid capacitor to charge same; trigger circuit means connected betweensaid capacitor and the gate electrode of said thyristor to cause saidthyristor to conduct when said capacitor is charged to a predeterminedpotential; and means connected to said capacitor and said source ofalternating current to provide a circuit path independent of saidcircuit means to discharge said capacitor at the end of each half cycleof potential applied across said thyristor from said source ofalternating current.

2. The apparatus of claim 1 wherein said trigger circuit means comprisesa trigger diode connected between said capacitor and said thyristor.

3. The apparatus of claim 1 wherein said means to discharge comprises atransistor connected between said capacitor and ground, and meansconnected to said source of alternating current to cause said transistorto be nonconductive except at the end of each half cycle of saidalternating current when the potential applied across said thyristor iszero, at which time said transistor is conductive.

4. The apparatus of claim 3 wherein said means to cause said transistorto be nonconductive comprises a resistor and a first diode connected inseries across said source of alternating current, means including asecond diode connected between the junction between said resistor andfirst diode and said transistor to cause said transistor to benonconductive when a potential of first polarity exists at saidjunction, and means including a second transistor connected between saidjunction and the first-mentioned transistor to cause saidfirst-mentioned transistor to be nonconductive when a potential ofsecond polarity exists at said junction.

5. The apparatus of claim 4 wherein each of said transistors has anemitter, a collector and a base, the base of said second transistor isconnected to said junction, said second diode is connected between thebase and collector of said second transistor, the emitters of saidtransistors are connected to ground, the collector of said secondtransistor is connected to the base of said first-mentioned transistor,and including means connecting the collector of said first-mentionedtransistor to said capacitor.

1. Temperature control apparatus comprising: an electrical heatingelement; a temperature-sensing element adapted to be positioned in aregion of temperature to be controlled; a source of alternating current;a bidirectional thyristor having a gate electrode; means connecting saidthyristor and said heating element in circuit with said source ofalternating current; a capacitor; circuit means including saidtemperature sensing element to establish a current, the magnitude ofwhich is representative of the temperature sensed by said sensingelement; means to apply the current established by said circuit means tosaid capacitor to charge same; trigger circuit means connected betweensaid capacitor and the gate electrode of said thyristor to cause saidthyristor to conduct when said capacitor is charged to a predeterminedpotential; and means connected to said capacitor and said source ofalternating current to provide a circuit path independent of saidcircuit means to discharge said capacitor at the end of each half cycleof potential applied across said thyristor from said source ofalternating current.
 2. The apparatus of claim 1 wherein said triggercircuit means comprises a trigger diode connected between said capacitorand said thyristor.
 3. The apparatus of claim 1 wherein said means todischarge comprises a transistor connected Between said capacitor andground, and means connected to said source of alternating current tocause said transistor to be nonconductive except at the end of each halfcycle of said alternating current when the potential applied across saidthyristor is zero, at which time said transistor is conductive.
 4. Theapparatus of claim 3 wherein said means to cause said transistor to benonconductive comprises a resistor and a first diode connected in seriesacross said source of alternating current, means including a seconddiode connected between the junction between said resistor and firstdiode and said transistor to cause said transistor to be nonconductivewhen a potential of first polarity exists at said junction, and meansincluding a second transistor connected between said junction and thefirst-mentioned transistor to cause said first-mentioned transistor tobe nonconductive when a potential of second polarity exists at saidjunction.
 5. The apparatus of claim 4 wherein each of said transistorshas an emitter, a collector and a base, the base of said secondtransistor is connected to said junction, said second diode is connectedbetween the base and collector of said second transistor, the emittersof said transistors are connected to ground, the collector of saidsecond transistor is connected to the base of said first-mentionedtransistor, and including means connecting the collector of saidfirst-mentioned transistor to said capacitor.